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>>>a new u.n. report out today warns the world must act now to address
climate change
to avert disaster. details from our chief environmental affairs correspondent
anne thompson
.

>> reporter: the report says time is running out to avoid the worst impacts of
climate change
. melting
ice sheets
that will raise
sea levels
and swamp coastlines. stronger heatwaves and droughts that could put the world's
food supply
at risk, particularly the basic crops of wheat and corn. the u.n. panel says the world must act now.

>>we wait for more than about 10 or 15 years, we really will make it extremely difficult for us to keep climate from changing substantially, and really, exposing ourselves to some substantial harms.

>> reporter: to protect itself, the report says the world must reduce
greenhouse gas emissions
40 to 70% by the year
2050
and be near zero by 2100. those are the emissions we create
burning oil
, coal, and gas for power and transportation. in its place, the report calls for a
low carbonenergy supply
, tripling renewable power, solar, wind, and geothermal, more
nuclear power
. coal only if carbon is captured stored, or instead of coal, cleaner burning
natural gas
, a bridge to a greener future.

>>it's like an
insurance policy
because we don't have the answers to everything other than that we know there is a tremendous risk.

>> reporter: but it may not be enough. the report warns by the second half of this century, we may need technologies to pull
carbon dioxide
out of the atmosphere. this video illustrates one company's idea.

>>we're trapping over 100 kilos of co2 here every day.

>> reporter: currently, only forests remove
carbon dioxide
from the air on a large scale. the report acknowledges all of this has a price, but not one that would sink the world's economy.

>>that's absolutely the case. and all the scenarios that are explored in this report, we still find robust economic growth.

>> reporter: the cost and difficulty will only increase, says the u.n., the longer the
world waits
to act.
anne thompson
, nbc news, detroit.